1. Field of the Invention
This invention relates to a method for enhancing and controlling the solubility of a lithium salt in liquid sulfur dioxide. More particularly, it relates to a method for increasing the solubility of lithium salts in a sulfur-dioxide based solvent system which involves the addition of a solubility enhancing additive comprising a salt which contains at least one cation selected from the group consisting of metal cation complexes, quaternary ammonium cations and organic phosphonium cations.
2. Description of the Prior Art
Recently much effort has been expended in the development of ambient temperature, high energy density cell systems which provide both higher voltages and total capacity (volumetric and gravimetric) than those of the Leclanche or alkaline cells having zinc anodes. The high energy density cell systems are centered around the use of active metals (metals above hydrogen in the EMF series which are unstable in aqueous environments) as anodes in non-aqueous solution cells. As used herein, "non-aqueous" is intended to mean substantially anhydrous.
High energy output and low weight requires the use of active metals as the electrodes and, in particular active metals having low equivalent weights. The alkali metals qualify in these respects. However, since the alkali metals are generally reactive in electrolyte systems containing water, acids, alcohols, or other proton donating solvents, these liquids must be replaced by solvents incapable of undergoing protolytic reactions with such metals. A continuing problem in the choice of such solvents is the tendency to co-reduction of said solvents during the reduction of alkali metal ions to free metal. Thus, it is apparent that not only must the solvent be miscible with, and promote the electrical conductivity of, the supporting electrolyte salt, but it must also remain substantially chemically inert to the electrodes and electrolyte during storage as well as during operation of the electrochemical system. In like manner, the electrolyte must be chemically inert with respect to the electrodes.
Various cell systems have been developed utilizing lithium as the anode electrode material. The ones showing promise in terms of voltage stability and high discharge capability are those having fluid cathode depolarizers which also generally serve the function of supporting electrolyte salt solvent. When a cell of this type is not being discharged, the fluid depolarizer/supporting electrolyte solution reacts with the anode metal to a limited extent with a protective film coating being formed on the surface of the anode. Full reaction between the anode and fluid depolarizer with which it is in contact is thereby substantially prevented and cell self-discharge is limited.
To date, active metal battery art has uniformly emphasized the necessity for soluble anode products and insoluble cathode products during discharge, and the concomitant need to facilitate anodic (active metal) ion transport through the electrolyte to the cathode or positive electrode current collector. As recognized in the art, such systems employ anodes which are termed electrodes of the first kind. These are understood to be electrodes in which the potential determining ion in solution is not in equilibrium with a solid salt phase. Such cells are characterized by cell solutions which are unsaturated in the electrode (or potential determining) ion--allowing the concentration of the potential determining ion to be varied at will. As used in the art, such cells are further characterized by (a) very high (although less than saturated) concentrations of the anode ion; (b) soluble discharge products at the anode; and (c) high ion transfer in solution from anode to positive electrode.
For example, Gabano, (U.S. Pat. No. 3,511,716) discloses a cell in which oxidized lithium goes into solution during discharge and migrates toward the positive electrode. Gabano emphasizes that the solubility of lithium in the solution must be as high as possible to achieve adequate transfer of lithium ions to the positive electrode. Similarly, Skarstad et al. (U.S. Pat. No. 4,246,327) discusses SO.sub.2 and SOCl.sub.2 based batteries which make use of lithium ion transport and deposit lithium salts as insoluble discharge products on a high surface area cathode.
More specifically, the art has taught that soluble anode products coupled with insoluble cathode products are required and necessary in active metal secondary batteries. Maricle et al. (U.S. Pat. No. 3,567,515) for example, simply state "As a general rule, insoluble [cathode] products are obtained when alkali-metal electrolytes are employed . . . ". In addition, Maricle et al. expressly teach that soluble cathode products are not preferred, especially in secondary batteries.
An even more sweeping generalization is found in Eisenberg, "Study of the Secondary lithium Electrode in Organic Electrolyte", Final Report on LBL Subcontract 4507210, April 1981. Eisenberg teaches (at page 19) that, for a lithium anode in organic, aprotic electrolyte systems, high solubility of lithium ion salts is necessary to provide for the necessary ionic transport through the electrolyte. In such systems, Eisenberg concludes that " . . . the solubility of the lithium anode product in the electrolyte appears to be an unavoidable fact of life." (emphasis in original).
Though the art has thus concentrated exclusively upon active metal anodes of the first kind, the hoped-for advantages of greater conversion of the active metal, higher effective current density and improved low temperature behavior have not been practically realized. Instead, a range of problems has plagued virtually all systems employing an active metal anode of the first kind. One such problem results from difficulties in obtaining requisite ionic transport to and from each electrode. During each charging operation, the active material must be transported from within the solution to the surface of the anode base plate. On the other hand, during each discharge the deposited layers must be completely redissolved. Maintaining uniform current distribution and adequate convection in the electrolyte is thus very important with this type of battery. If uniformity of current distribution is inadequate, several problems arise:
1. Local problems: formation of dendrites and nodules on the surface of the anode; PA0 2. Asymmetrical deposition of the active materials along the electrode surface (shape change); and PA0 3. Asymmetrical deposition of active materials in the positive electrode.
Additional problems arise from the partially irreversible behavior of the active deposits. As a result of the asymmetry between current efficiencies for deposition and dissolution, and as a result of asymmetry in respect of corrosion processes, the active deposits show a partially irreversible behavior. Dendrite formation in cells with anodes of the first kind and the attendant problems are discussed in Beck et al. (U.S. Pat. No. 4,119,767), by Koch (U.S. Pat. No. 4,252,876), and by Schlaikjer (U.S. Pat. No. 4,139,680). Beck et al. and Koch also encountered shape change or morphology problems. The disadvantages of irreversible asymmetrical deposits clogging the cathode are discussed by Beck et al., Fraioli et al. (U.S. Pat. No. 3,551,205) and Maricle et al.
In general, attempts to solve such problems associated with an active metal anode of the first kind have centered on (1) electrolyte-solvent combinations such as those taught by Gabano et al.; Skarstad et al.; and Eisenberg (U.S. Pat. No. 3,891,458, reissued as Res. No. 30,661); (2) additives to help dendrite dissolution such as the teachings of Beck et al.; and (3) high surface area cathodes such as those discussed by Maricle et al. and Fraioli et al.
To the best of our knowledge, no complete solution to the difficulties inherent in active metal anodes of the first kind has yet been found. This failure has prevented the development of an active metal secondary battery which is lightweight, provides stable voltage at open circuit, is operable at ambient temperatures, has large energy density, and can be reliably cycled through numerous charge/discharge cycles. A need exists for a secondary battery with the desirable characteristics expected of an active metal-based electrochemical cell, but without the present disadvantages.